Fire-resistant glazing panels of this kind are known from the document WO 94/04355. In this case, the transparent layer preferably comprises alkali metal polysilicate hydrate having an SiO.sub.2 :M.sub.2 O ratio of greater than 4:1 (where M denotes at least one alkali metal) and each glass sheet is provided with a primer layer formed of an organofunctional silane. The polysilicate layer foams in the event of fire under the effect of the heat with the evaporation of the water, it becomes impermeable to heat radiation and forms, for a predetermined period, an effective protection against undesirable transition of heat. In these known fire-resistant glazing panels, the organofunctional silanes have to influence the adhesion in the sense of improving the adhesion of the polysilicate layer to the glass sheets under all conditions and they are chosen from this viewpoint.
In WO 94/04355, good adhesion between the polysilicate layer and the glass surfaces is stated to be necessary and has to be guaranteed throughout the lifetime of the fire-resistant glazing panel, because delaminations of the polysilicate layer are visible in the form of quality defects with a large surface area.
Fire-resistant glazing panels in which the fire-resistant layer placed between two glass sheets is formed of a saline hydrogel comprising an organic agent forming a gel, for example based on acrylamide and/or methylolacrylamide, are known from the documents EP 0 001 531 B1 and EP 0 590 978 A1. In such fire-resistant glazing panels, the faces of the glass sheets which are in contact with the hydrogel are again treated with a substance which improves the adhesion.
In this case, use is exclusively made, as substances which improve the adhesion, of hydrophilic organic substances based on silanes, on titanates or on zirconates which react, on the one hand, with the surface of the glass and, on the other hand, with the carbon-carbon double or triple bonds of the polymer forming the gel.
During tests of behaviour towards fire carried out with fire-resistant glazing panels with the structure cited above, that is to say comprising an intermediate layer formed of an alkali metal polysilicate, it turns out that, after a specific time, locally restricted points frequently develop in which the protective effect of the foaming polysilicate layer decreases, so that the glass sheet of the fire-resistant glazing panel facing away from the fire exceeds the allowed temperature at these points. The formation of these defects, which are excessively overheated (hot spots), causes parts of the polysilicate foam to escape from the layer and fall off in the flame furnace.